[corbanpro]

Summary

• Document discriminated-union JSON Merge Patch on AIP-134: kind always required, same-kind merge, kind switch with defaults, reject cross-kind keys
• Cross-link related guidance from AIP-126 where Update / merge-patch behavior is described

Test plan

• Skim AIP-134 and AIP-126 diffs for clarity and consistency with public BFF PATCH behavior

Note

Add discriminated-union merge-patch rules to AIP-134 and cross-link AIP-126

Docs-only change to the API improvement proposals.

• 0134.md now requires PATCH updates to discriminated-union fields to include kind. Matching kinds merge present members; a different kind replaces the variant, with product defaults used where available and required fields without defaults supplied by the client.
• Members invalid for the stated kind must be rejected with a clear client error naming the member and kind. A discriminated-union exception is added to the general recursive merge rules.
• 0126.md gains an Update (PATCH) section referencing AIP-134. Create, Replace, and response bodies keep full variant shapes in both documents.
• Behavioral Change: APIs following this guidance must reject union patches without kind and with members outside the stated kind.

^{Macroscope summarized 46a1aa5.}